Process for the preparation of formaldehyde

ABSTRACT

A process for preparing formaldehyde by oxidative dehydrogenation of methanol comprises passing a gas mixture (i) comprising 
     a) from 0.1 to 50% by volume of methanol, 
     b) from 0.1 to 30% by volume of oxygen, 
     c) from 0 to 50% by volume of nitrogen oxide and 
     d) from 0 to 60% by volume of water at from 150 to 800° C. through a phosphorus-doped silver catalyst fixed bed (a) and applying from 0.01 to 100 ppm by weight of phosphorus, based on the phosphorus-doped silver catalyst fixed bed, in the form of a finely divided phosphorus compound having a melting point or decomposition temperature of more than 500° C. (phosphorus compound P) to the phosphorus-doped silver catalyst fixed bed (a) per kg of methanol which is passed through in the form of the gas mixture (i) per cm 2  of the cross-sectional area of the phosphorus-doped silver catalyst fixed bed (a).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing formaldehyde byoxidative dehydrogenation of methanol.

2. Description of the Background

Processes for preparing formaldehyde by oxidative dehydrogenation ofmethanol over a catalyst fixed bed made up of silver crystals aregenerally known (cf. Ullmanns Enzyklopadie der technischen Chemie, 3rdEdition, Urban und Schwarzenberg, Munich/Berlin, 1956, Volume 7, pages660 to 663).

Advantageous effects which occur when using phosphorus compounds aspromoters for the oxidation of methanol to give formaldehyde in thepresence of a silver catalyst are also known from CN-A-85100530,DE-A-4022603 and JP-A-38227/83.

EP-A-0 467 169 describes the preparation of formaldehyde by oxidativedehydrogenation of methanol over a catalyst fixed bed which is made upof layers of silver crystals containing a pulverulentphosphorus-containing salt as promotor. This phosphorus-doped silvercatalyst fixed bed is prepared by bringing it into contact with aphosphorus compound prior to the commencement of the formaldehydepreparation.

However, these processes are still capable of improvement because thecatalyst continuously loses activity during use for the formaldehydepreparation, which is reflected in a decreasing yield. For this reason,the catalyst has to be replaced and regenerated after a relatively shorttime, for which purpose the formaldehyde preparation process has to beinterrupted. Since the replacement of the catalyst fixed bed and themeasures associated with the resumption of the formaldehyde preparationprocess are time-consuming and laborious, the economics of the processdepend on how frequently this replacement has to be carried out.

SUMMARY OF THE INVENTION

It is an object of the present invention to develop a process in whichthe time between the required replacement of the catalyst is prolonged.

We have found that this object is achieved by a process for preparingformaldehyde by oxidative dehydrogenation of methanol, which comprisespassing a gas mixture (i) comprising

a) from 0.1 to 50% by volume, preferably from 10 to 40% by volume, ofmethanol,

b) from 0.1 to 30% by volume, preferably from 5 to 20% by volume, ofoxygen,

c) from 0 to 50% by volume, preferably from 1 to 20% by volume, ofnitrogen oxide and

d) from 0 to 60% by volume, preferably from 10 to 50% by volume, ofwater

at from 150 to 800° C. through a phosphorus-doped silver catalyst fixedbed (a) and applying from 0.01 to 100 ppm by weight of phosphorus, basedon the phosphorus-doped silver catalyst fixed bed, in the form of afinely divided phosphorus compound having a melting point ordecomposition temperature of more than 500° C. (phosphorus compound P)to the phosphorus-doped silver catalyst fixed bed (a) per kg of methanolwhich is passed through in the form of the gas mixture (i) per cm² ofthe cross-sectional area of the phosphorus-doped silver catalyst fixedbed (a).

DETAILED DESCRIPTION OF THE INVENTION

These data are based on a pressure of 1 bar.

The gas mixture generally contains from 0.25 to 0.60, preferably from0.35 to 0.50, mol of oxygen per mol of methanol and from 0.2 to 3.0,preferably from 0.67 to 1.75, mol of water per mol of methanol and from0.9 to 2.3, preferably from 1.3 to 1.8, mol of nitrogen per mol ofmethanol.

Phosphorus-doped silver catalysts suitable for the process of thepresent invention are, for example, those obtainable by

I. arranging silver crystals which are obtained by electrolyticdeposition of silver from an aqueous silver salt solution to form astarting silver catalyst fixed bed (a), and subsequently

II. bringing the starting silver catalyst fixed bed (a) into contactwith from 1 to 20,000 ppm by weight of phosphorus, based on the silver,in the form of the phosphorus compound (P) before the gas mixture (i) ispassed at from 150 to 800° C. through the fixed bed.

The preparation of the silver crystals described in step I is generallyknown (cf. Ullmann's Enzyklopadie der technischen Chemie, 3rd Edition,Urban und Schwarzenberg, Munich/Berlin, 1956, Volume 7, pages 660 to663). Particularly good results can be achieved using the startingcatalyst fixed beds described in DE-A-2322757.

Suitable silver crystals are obtained particularly when the electrolysisis carried out according to the process described in the German Patent1166171.

As electrolyte, preference is given to using an aqueous silver nitratesolution. This silver nitrate solution generally has a pH of from 1 to 4and contains from 1 to 5% by weight of silver. The pH is advantageouslyadjusted using nitric acid.

The electrodes employed are those customarily used in the electrolysisof silver. Suitable anodes are sacks which have been charged with thesilver to be oxidized, generally as granules or powder. Suitablecathodes are, in particular, silver sheets.

The electrolysis is advantageously carried out at current densities offrom 80 to 500 A/m² of cathode area and electrolyte temperatures of from10 to 30° C.

To achieve these current densities, voltages of from 1 to 15 volts arenecessary in most electrolysis cells.

It is advisable to continually remove the silver crystals formed fromthe cathode. Silver crystals having a particle size of from 0.2 to 5 mmare generally obtained.

A single electrolysis is usually sufficient to obtain usable silvercrystals.

In general, the silver crystals are arranged to form a starting silvercatalyst fixed bed (a) which comprises from 1 to 9 layers of silvercrystals and has a total bed thickness of from 1 to 10 cm. Such fixedbeds which are also designated as short beds are generally known (cf.Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, VerlagChemie, Weinheim-N.Y., Volume 13, pages 539 to 541).

In step II, the starting silver catalyst fixed bed (a) is brought intocontact with from 1 to 20,000 ppm by weight, preferably from 5 to 5000ppm by weight, of phosphorus, based on the silver, in the form of afinely divided phosphorus compound having a melting point ordecomposition temperature of more than 500° C. (phosphorus compound P).

Suitable phosphorus compounds (P) are phosphorus-containing salts.Examples of these are the phosphorus-containing salts mentioned inDE-A-4022603, e.g. inorganic phosphates of alkali metals, alkaline earthmetals and heavy metals such as Ag, Zn and Fe or of boron and ammonium.

Preference is given to phosphates or pyrophosphates of alkali metals oralkaline earth metals, e.g. Na₄ P₂ O₇, Li₃ PO₄, Mg₃ (PO₄)₂, Ca₃ (PO₄)₂.

In general, the procedure is to sprinkle a finely divided powder of thephosphorus compound (P) onto the silver catalyst fixed bed or toimpregnate the silver catalyst fixed bed with a solution of thephosphorus compound (P) and to allow the solvent to evaporate.

The particle size of the phosphorus compound (P) used as powder is notcritical; it is generally from about 1 mm to 1 μm. The solutions of thephosphorus compounds (P) are generally aqueous solutions containing from0.01 to 50% by weight of the phosphorus compound (P). To impregnate thesilver catalyst fixed bed, it is soaked with one of the solutions or,particularly advantageously, the solutions are sprayed onto the silvercatalyst fixed bed to be activated and the solvent is subsequentlyevaporated.

The amount of phosphorus compound (P) sprayed on or sprinkled on ispreferably selected such that the amount of phosphorus is from 0.01 to100, preferably from 0.05 to 10, mg per cm² of the cross-sectional areaof the phosphorus-doped silver catalyst fixed bed.

The silver catalyst fixed beds (a) produced in this way generally do notdisplay their full catalytic activity at the commencement of the passingthrough of the gas mixture (i). It has therefore been found to beadvantageous to activate the phosphorus-doped silver catalyst fixed beds(a) at the commencement of the formaldehyde preparation.

The activation of the catalyst can, for example, be carried out bypreheating the catalyst to from 300 to 400° C. immediately before thecommencement of the passing through of the gas mixture (i) and/orpassing a gas mixture (i) preheated to from 100 to 800° C., preferablyfrom 200 to 700° C., through the fixed bed. The amount of methanol inthe form of the gas mixture (i) which is passed through thephosphorus-doped silver catalyst fixed bed (a) at the commencement ofthe activation phase, which usually takes from 0.1 to 100 hours, perhour and per cm² of cross-sectional area of this silver catalyst fixedbed is from 0.001 to 1 kg. The amount of gas mixture (i) which is passedper unit time through the phosphorus-doped silver catalyst fixed bed (a)is continuously increased during the activation phase to the final valuewhich is generally from 0.1 to 1 kg. In general, the preheating of thegas mixture (i) becomes superfluous at the latest after the end of theactivation phase, since the fixed bed is heated to the requiredtemperature by the heat of reaction which is liberated.

To enable the activation phase and the preparation of formaldehyde bythe process of the present invention to follow one another without abreak, the activation of the starting catalyst fixed bed (a) isadvantageously carried out in a fixed-bed reactor as is customarily usedfor the preparation of formaldehyde by oxidative dehydrogenation ofmethanol and the gas mixture (i) is continuously passed through this.The reactor is here preferably vertical and the gas mixture (i) ispassed through the reactor from the top downward. Such reactors andprocesses are described, for example, in EP-A-467 169, DE-A-2444586 andEP-A-0150436.

The cross-sectional area of the reactor and the starting silver catalystfixed bed (a) are advantageously selected so as to be the same and thecatalyst fixed bed is arranged in the reactor so that the layers of thesilver crystals are perpendicular to the flow direction of the gasmixture (i).

The phosphorus-doped silver catalyst fixed beds (a) have their activitymaximum at the end of the activation phase and slowly and steadily loseactivity during use in the process of the present invention, whichbecomes apparent from a falling yield of formaldehyde.

This activity loss for the phosphorus-doped silver catalyst fixed beds(a) can be partially avoided if from 0.01 to 100 ppm by weight ofphosphorus, based on the phosphorus-doped silver catalyst fixed bed (a),in the form of the phosphorus compound (P) per kg of methanol in theform of the gas mixture (i) which, based on 1 cm² of the cross-sectionalarea of the phosphorus-doped silver catalyst fixed bed, is passedthrough the latter, is applied to the silver catalyst fixed bed, eithercontinuously or discontinuously (in each case in one portion afterintroduction of a defined amount of gas mixture (i)), preferably withoutinterrupting the introduction of the gas mixture (i). In the case ofcontinuous application, the activity loss can be slowed down; in thecase of the stepwise, discontinuous application, the activity loss canbe partially reversed.

If the subsequent application of the phosphorus compound (P) is carriedout discontinuously, the intervals between the applications of thephosphorus compound (P) to the phosphorus-doped silver catalyst fixedbed are selected such that during this time not more than 500,preferably from 1 to 5, kg of methanol in the form of the gas mixture(i), based on 1 cm² of the cross-sectional area of the phosphorus-dopedsilver catalyst fixed bed, are passed through the catalyst fixed bed,since otherwise the yield would drop too much in the meantime.

Otherwise, the preparation process for formaldehyde by oxidativedehydrogenation of methanol using the catalyst fixed bed of the presentinvention is carried out in a manner known per se, by passing the gasmixture (i) at from about 500 to 750° C., in particular from 600 to 710°C., through the phosphorus-doped silver catalyst fixed bed. The processis generally carried out continuously at a pressure of from 0.5 to 2bar, preferably from 0.8 to 1.8 bar. It is here advantageous to cool thereaction gases leaving the catalyst zone within a short time, forexample to from 50 to 350° C.

Preferably, the phosphorus-doped silver catalyst fixed bed is located ina vertical reactor and the gas mixture (i) is passed through the reactorfrom the top downward. Advantageously, the cross-sectional area of thereactor and that of the starting silver catalyst fixed bed (a) areselected so as to be equal and the catalyst fixed bed is arranged in thereactor so that the layers of the silver crystals are perpendicular tothe flow direction of the gas mixture (i).

The cooled gas mixture is then advantageously conveyed to an absorptiontower in which the formaldehyde is scrubbed from the gas mixture usingwater or an aqueous formaldehyde/urea solution.

Specific advantageous variants of the generally known process forpreparing formaldehyde which can also be employed in the process of thepresent invention are recommended in DE-A-2444586, DE-A-2451990,EP-A-0083427 and EP-A-0150436.

The process of the present invention is notable, in particular, for thefact that it enables formaldehyde to be prepared particularlyeconomically, because in this process the yield and the selectivity inthe oxidative dehydrogenation of methanol is particularly high over along period of time.

EXAMPLE 1

In a vertical experimental reactor having an internal diameter of 15 cm,a three-layer starting silver catalyst fixed bed having the samediameter and a total bed thickness of 2 cm was installed . The lowerlayer comprised 1000 g of silver crystals having a particle size of from1 to 2.5 mm, the middle layer comprised 65 g of silver crystals having aparticle size of from 0.75 to 1 mm and the upper layer comprised 185 gof silver crystals having a particle size of from 0.2 to 0.75 mm.

Phosphorus in the form of pulverulent Na₄ P₂ O₇ was sprinkled on thesurface of the staring silver catalyst fixed bed in an amount of 1.3 mgof phosphorus (calculated as elemental phosphorus) per cm² ofcross-sectional area of the fixed bed. The phosphorus-doped silvercatalyst fixed bed thus produced was heated to 360° C. Subsequently, toactivate the catalyst, a gas mixture comprising methanol, air and waterwas passed through the catalyst. During the 28-hour activation period,the amount was increased to 32 kg of methanol, 21.4 kg of water and 54kg of air per hour (final throughput). The temperature in the fixed bedwas 700° C. at the end of the activation period. This feed flow was keptconstant during the entire time of the experiment.

During the continuous operation of the reactor, further amounts inphosphorus in the form of pulverulent Na₄ P₂ O₇ were applied to thephosphorus-doped silver catalyst fixed bed after the operating timesindicated in Table 1 (operating day 0 is the point in time immediatelyafter the activation of the catalyst). The respective amounts are givenin Table 1 (cumulative amounts).

                  TABLE 1                                                         ______________________________________                                        Operating    Amount of phosphorus                                             days         [g/cm.sup.2 ] Yield [%]                                          ______________________________________                                        0            1.3           90.1                                               4            1.3           89.7                                               6            1.42          90.1                                               9            1.42          89.6                                               11           1.61          90.0                                               16           1.61          89.3                                               17           1.70          89.9                                               ______________________________________                                    

We claim:
 1. A process for preparing formaldehyde by oxidative dehydrogenation of methanol, which comprises passing a gas mixture (i) comprisinga) from 0.1 to 50% by volume of methanol, b) from 0.1 to 30% by volume of oxygen, c) from 0 to 50% by volume of nitrogen oxide and d) from 0 to 60 % by volume of water at from 150 to 800° C. through a phosphorus-doped silver catalyst fixed bed (a) and applying from 0.01 to 100 ppm by weight of phosphorus, based on the phosphorus-doped silver catalyst fixed bed in the form of a finely divided phosphorus compound (P) having a melting point or decomposition temperature of more than 500° C., to the phosphorus-doped silver catalyst fixed bed (a) per kg of methanol which is passed through in the form of the gas mixture (i) per cm² of the cross-sectional area of the phosphorus-doped silver catalyst fixed bed (a).
 2. A process as claimed in claim 1, wherein the phosphorus-doped silver catalyst fixed bed (a) used is obtainable byI. arranging silver crystals which are obtained by electrolytic deposition of silver from an aqueous silver salt solution to form a starting silver catalyst fixed bed (a), and subsequently II. bringing the starting silver catalyst fixed bed (a) into contact with from 1 to 20,000 ppm by weight of phosphorus, based on the silver, in the form of the phosphorus compound (P) before the gas mixture (i) is passed at from 150 to 800° C. through the fixed bed.
 3. A process as claimed in claim 1, wherein the phosphorus-doped silver catalyst fixed bed (a) used is made up of one or more layers of silver crystals whose longest mean diameter is from 0.2 to 10 mm and the total bed thickness is from 1 to 10 cm.
 4. A process as claimed in claim 1, wherein the phosphorus compound (P) used is a phosphate or pyrophosphate of an alkali metal or alkaline earth metal. 